prop. n. [ F. ] (Bot.) A genus of algae. The plants are composed of moniliform cells imbedded in a gelatinous substance. [ 1913 Webster ]
☞ Nostoc commune is found on the ground, and is ordinarily not seen; but after a rain it swells up into a conspicuous jellylike mass, which was formerly supposed to have fallen from the sky, whence the popular names,
prop. n. A natural family of blue-green algae.